Blood Spatter — Evidence?

Well … you knew that sooner or later we’d get around to “blood spatter”. This is also sometimes called “blood pattern analysis” (BPA). As the name implies, BPA is the analysis of patterns made by blood that has been expelled from the body by a violent act – stabbing, shooting, beating. These patterns can be used to help reconstruct the violent event, and can provide information about type of injury sustained, movement of a victim, angle of a shooting, location of attack, etc. Unfortunately, BPA is one of the forensic disciplines least based upon real science. It has evolved from a collection of anecdotal and empirical observations that have resulted in potentially flawed inductive inferences about the fluid dynamic behavior of blood outside the body.

Now, like all forensics, BPA is not useless. There is certainly legitimate information that can be garnered from an analysis of blood patterns at a crime scene. But, again, as with all forensics, it must be used within the bounds of what is scientifically supportable.

I’ve heard BPA experts speak, and they try to dress it up as science; even citing “experiments” that they say support their theories. Without going into a lecture about the ‘scientific method’ and ‘design of experiments’, I can say that none of the “experiments” I’ve seen have been properly controlled scientific experiments conducted with a single independent variable and rigorous hypothesis testing. For example, anyone familiar with fluid dynamics can tell you that one of the major variables determining the behavior of a fluid is viscosity. Did you know that the viscosity of blood can vary by a factor of 2-to-1 depending on the hematocrit level (volume concentration of red blood cells)? I am not aware of any BPA experiments ever done that have controlled for hematocrit level.

The results of the simple experiments that have been done are taken by BPA “experts” and extrapolated way beyond what is scientifically supportable – and this results in wrongful convictions.

A classic example of BPA gone wrong is the case of Warren Horinek in Ft.Worth, TX in 1995. It’s rare that BPA is used as the sole evidence for conviction, but here’s a case where that happend. Below is a link to a Texas Observer article detailing the case. As you will read, it’s a bizarre case in which even the DA and the investigating officers testified for the defense, but blood spatter testimony from an independent “expert” resulted in a conviction – and it was wrong.

The NAS Report Ch.5 presents a summary evaluation of many of the forensic disciplines and BPA is covered on pages 177-179. Here is just an excerpt from that section: “In general, the opinions of bloodstain pattern analysis are more subjective than scientific. In addition, many bloodstain pattern analysis cases are prosecution driven or defense driven, with targeted requests that can lead to context bias.” And, “The uncertainties of bloodstain pattern analysis are enormous.“

The David Camm case is another like the Horinek case. He was convicted twice on BSA and in the second trial it was the same “expert” as the Horinek case. Both convictions were overturned on appeal but he still remains in prison awaiting a 3rd trial. The thing about the case – they found foreign DNA at the murder site and matched it to a career criminal. He was tried and convicted of the murders. They still hold onto Camm, saying it could have been a conspiracy, though no link exists. The most amazing thing is he has an airtight alibi – he was playing basketball at his church gym with 11 other people at the time of the murders. If not for the BSA, they really couldn’t possibly hold him because there is nothing. I’ll be following the case closely. My article on the alibi and BSA in this case:http://stopwrongfulconvictions.wordpress.com/2012/02/23/david-camms-alibi/